In the design of high-speed interconnects, the level of unwanted electrical crosstalk between adjacent signals is one metric used when characterizing communication channels. In practice, this crosstalk is a portion of overall noise, which normally results in increased bounded uncorrelated jitter (BUJ). In a printed circuit board (PCB) one source of crosstalk inherently exists in a via field beneath a typical integrated circuit (IC). As is well recognized, one common structure used to provide connectivity to the IC is a ball grid array (BGA) which is used to connect signal contacts on a bottom side of the IC to related structures within the PCB. This requires a grid-like via field as part of the PCB, which is appropriately positioned and sized to cooperate/communicate with the IC. A number of solutions exist to minimize crosstalk within this via field structure, including increasing in the separation between signals (i.e. employing larger via pitch and trace spacing), and adding ground vias or guard traces between signals.
It is understood that crosstalk within a via field is generally related to the length of the associated via barrel, which typically extends from top to bottom of the PCB. A thicker PCB is expected to create a higher lateral near-end and far-end crosstalk (NEXT and FEXT) from longer vias which are located nearby (either to the left or right, or diagonally below). The level of crosstalk is proportional to the length of the adjacent vias.
In recently adopted PAM4 multi-level signaling applications, having communication channels operating at very high data rates (56 Gbps), crosstalk plays a more critical role in the signal integrity of the channel. Industry experts agree that these high-speed channels becomes more sensitive to crosstalk and other impairments, as compared to more conventional non-return to zero (NRZ) data communication signaling. Crosstalk (NEXT and FEXT) can degrade these advanced data channels by a factor of 6 to 9 according to industry experts, a direct result of employing this very sensitive multi-level PAM4 signaling. This is partly because of a greatly reduced (by ⅓) step-height, but is also a result of the large variation in rise and fall times of these multi-level signals, both factors which are inherent to the nature of PAM4 technology. As such designing a high-speed communication channel with minimized crosstalk is a key technology improvement, essential to the successful product development and implementation of modern high-speed multi-level signaling systems.